582 SUMMARY OF CURRENT RESEARCHES RELATING TO 



part of the silica in the glass was to reduce the dispersion of the blue 

 end of the spectrum and so to make a flint glass which more nearly 

 matched the ordinary crown glass. He was also able to modify the 

 crown glass by using phosphoric acid, but wrongly attributed the result 

 to the presence of titanic acid. Unfortunately these experiments did 

 not lead to practical results, probably because of the expense attaching 

 to experimental meltings on a practical scale. Schott and Abbe (1881- 

 1886) confirmed Harcourt's results as to the action of boric acid, and 

 correctly attributed the effects observed by him to phosphoric acid. In 

 addition they were able to determine the effects of barium both with and 

 without boric acid. Now the use of boric acid in the ordinary lead 

 glasses always leads to a glass which is more or less liable to spot, but by 

 the use of barium instead of some of the lead this effect is reduced. The 

 boric acid barium glass is, however, of special value, because in this case 

 a high refractive index is associated with a low dispersion. It behaves 

 as a crown glass as regards its dispersion, but as a flint glass in respect 

 of its refractive index. This property is of special value in the con- 

 struction of modern anastigmat photographic lenses. These researches 

 of Schott and Abbe were followed by the establishment of the Jena 

 glass-w r orks. The immediate results were : — 



1. The manufacture of flint glasses containing boric acid ; by the aid 

 of these glasses it was possible to make three lens objectives free from 

 secondary spectrum, but these glasses are not so permanent as the older 

 types. 2. The series of phosphate glasses whicli proved chemically un- 

 stable and deteriorated in use. 3. The boro-silicate crown glasses, which 

 are of somewhat lower refractive index and dispersion than the ordinary 

 crown glass. These are good glasses, and are now extensively used for 

 small objectives and for prisms in prism-binoculars. 4. The dense 

 barium crown glasses containing barium and boric acid. These glasses 

 are used in nearly all anastigmat photographic lenses, but they are 

 difficult to make because such abnormal optical qualities are closely 

 associated with chemical instability. 5. The most important result was, 

 however, the possibility of obtaining a large range of refractive index 

 and dispersion, so that the designer was able to regard the dispersion and 

 refractive index as more or less independent of each other. 



These great successes led at first to a concentration of the optical 

 glass industry in Jena, but the success of Chance in improving the 

 quality of the older types of glass, and of Mantois in making the newer 

 types, have somewhat modified this situation, though we are still 

 dependent on Jena for some of the special glasses. 



